The present invention relates to a noise reduction signal processing apparatus for reducing noise components included in television video signals; and, more particularly, the invention relates to a noise reduction signal processing apparatus for reducing noises using a median filter.
Conventional techniques used in noise reducing apparatus for reducing noises in video signals using a median filter are disclosed, for example, in "Proposal of a Direct-Current Addition Type Noise Reducer and Development of a MUSE Decoder Dynamic Picture Image Processing Noise Reducer" (Izumi and four others, academic circle magazine of television, Vol. 48, No. 12, pp 1553-1564 (1994); and in the official gazette of Unexamined Published Japanese Patent Application No. 9-163185. This apparatus is referred to as a DC-shifting type NR (Noise Reducer).
Hereunder, a description will be provided concerning the operation and characteristics of the median filter used for such a noise reducing apparatus. This median filter receives input digital video signal data sampled at a desired time and digital video signal data sampled at two points, which are separated from each other in the direction of time according to the desired lengthwise symmetrical or asymmetrical sampling counts. The median filter then outputs video signal data detected at those three different sampling times.
Since the median filter can smooth small amplitude signals more effectively than low-path filters, noises in these output signals can be reduced effectively if small amplitude noise waves are included in those large amplitude video signals. The median filter can therefor pass those large amplitude signals detected at precipitous rising and falling portions without losing such precipitous shapes and timings. Consequently, the signals output from this median filter can be assumed to be ideal original video signals including no noise, whereby those signals are referred to as reference signals.
This noise reducing apparatus compares input signals with the voltage levels of respective reference signals and judges the difference between the voltage level of each input original signal and the voltage level of each reference level, so as to get the noise included input signals close to the reference signals. And, as a result of such judgment, if the voltage level of the input signal is larger than the voltage level of the reference signal, a DC value equivalent to a noise reduction value to be described later is subtracted from the original signal value, thereby obtaining a signal, which becomes a noise-reduced video signal. On the contrary, if the voltage level of the input original signal is smaller than the voltage level of the reference signal, a DC value equivalent to a noise reduction value to be described later is added to the original signal, thereby obtaining a signal, which becomes a noise-reduced video signal. If the voltage level of the original signal is equal to the voltage level of the reference signal, the original signal is output as it is.
The DC value described above is a noise reduction value corresponding to an effective voltage, which is an average level of a noise. This is obtained by detecting a noise component in the synchronizing signal, etc., where there is no video signal, such as in the vertical blanking period, in the input video signals which are original signals, and then integrating the component.